Just one note: pay attention to the insulation on the wire. Using general
pvc insulated wire will cause a leak itself. Best would be to embed some
stainless bolts, and connect wires to them using nuts and washers on the
exterior side.
Vajk
On Sun, Jul 7, 2013 at 9:18 PM, Jerry Scharf <[email protected]
> wrote:
> On 07/06/2013 08:40 AM, Peter Hollenbeck wrote:
> > I am building a 26 foot wood power boat. The bilge below the cabin and
> > afterdeck floor is 7 sealed water tight (hopefully) compartments. I
> > plan no inspection ports but would like a way to detect moisture. If
> > there were an appropriate sensor I could embed one in each compartment
> > and occasionally hook up a Raspberry Pi and check for the presence of
> > water. Cost is a consideration.
> >
> > I would appreciate suggestions.
> > Thanks,
> > Peter
> >
> >
>
> Peter,
>
> If I understand the job right, doesn't that mean that you will be doing
> major surgery to dry the compartment and solve the water ingress issues?
> If so, I think you could just go with a pair of bare copper wires spaced
> 1/2 inch apart near the back bottom part of the compartment (replacing
> the wire is the least of your problems if it gets wet.) (Make sure you
> use separate pieces of tape for the two wires.)
>
> You have one wired to ground and the other that is pulled to a fixed
> voltage with a pair of high resistance resistors (~10^5 range) with one
> to supply voltage and the one to ground. With no conductance, the wire
> will stay at the voltage of the resistor voltage divider, with water in
> the bottom it will drop toward ground. This can be measured with just
> about any of the voltage sensing 1-wire chips. I would have the
> resistors and the sense chip attached to the pi and not to the wires. I
> would just solder the lengths of copper to insulated, waterproof wire
> and bring them to a small dry box. In the box you would tie all the
> grounds together and bring the sense wires to test contacts.
>
> You take a first reading for each sense line and record the voltage.
> When you take follow-on readings, you just need to make sure that the
> bottom of the boat is tilted so that any water sits at the back of each
> compartment (I think this is usually the case in motor boats.) Then you
> connect up a lead, give the sense wire a little while to charge up and
> take the reading. There may be some systemic error do to
> time/temperature variations in the system and resistors, This can be
> factored out by taking a reading with no sense wire attached. It's
> ability to sense moisture is limited, but significant moisture in the
> wood would change the resistance between the wires at least somewhat.
>
> YMMV
>
> jerry
>
>
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